Based on your description it is impossible to make a definite diagnosis. I would suggest you keep a close eye on them, and next time you see a branch just beginning to go, consult with your county extension (250-1100) as to the cause. In the meantime, remove any dead branches and destroy them to limit the chance of reinfection in case it is an insect or disease. Examine these closely for any unusual signs such as bark damage and again, consult with your county extension if you can see anything unusual beyond the fact that it is dead. I'm really sorry I can't be more helpful.